The instrumentation of geothermal wells requires development of electronic circuits for well-logging tools. Such circuits must be able to withstand environmental conditions generally much more severe than are encountered by conventional or military electronics. Components suitable for utilization in the instrumentation of geothermal wells are required to maintain their stability at temperatures up to about 500.degree. C. Instrumentation for other high-temperature applications, such as nuclear reactors, also requires high-temperature stable components.
Useful electronic circuits require a combination of both passive components and active devices. This means that passive components, such as resistors, which meet the high-temperature stability requirements, must also be compatible with active components both in fabrication processing and in circuit performance. Thus what is needed is a thin-film technology which is compatible with the processing methods used in fabricating either semiconductors or integrated thermionic devices and will produce passive components which are electrically compatible with these active devices.
Resistors for use in such circuits must have a well characterized resistivity and temperature coefficient of resistance, hereinafter referred to as TCR. The TCR is a measure of the change in resistance with respect to the change in operating temperature of the resistor and is commonly expressed in units of ppm/.degree.C. A resistor with a positive, negative or zero TCR may be required depending on the type of circuit and its particular applications. Moreover, the bulk and sheet resistivities of these components should be as independent of the TCR as possible. Thus, the method of fabricating thin-film resistors in addition to the requirements mentioned above, must be able to produce resistors of preselected resistivity and TCR.
Present processes for preparing thin-film resistors include the plating of nickel-chromium on a substrate by the thermal evaporation of nickel-chromium compounds in a vacuum or by the ion-bombardment of a nickel-chromium target. While thin films of nickel-chromium and also tantalum have found acceptance, they are not capable of withstanding temperatures greater than about 125.degree. C. and thus are unsuitable for applications which require high-temperature stability.
U.S. Pat. No. 3,563,873 to D. S. Beyer describes a method for producing thin-film tungsten-silicon resistors suitable for high-temperature applications by sputter deposition from a tungsten-silicon target onto a silicon substrate. The sputter deposition method does not provide for strict control of the resistivity of the final product because the resistivity depends on the elemental composition of the deposited material and that composition can not be predicted from the composition of the ion-bombarded target. Furthermore, there is no control over the temperature coefficient of resistance (TCR) of the sputtered material which is necessary for full integration of electronic functions. In order to change the composition of a sputtered film a target of a different composition must be used. The high cost of these targets restricts the range of tungsten to silicon ratios of the final resistors. U.S. Pat. No. 3,540,920 to G. F. Wakefield describes a method of producing oxidation-resistant coatings by vapor deposition of silicon and tungsten onto metal alloy substrates. This invention, because it is directed to the non-analogous art of coatings rather than thin-film resistors, does not disclose a method of fabricating resistors of predetermined resistivity and TCR which are compatible with other electronic components for use under high temperature conditions.